Eminent British historian Orlando Figes yesterday accused the Russian authorities of trying to 'rehabilitate the Stalinist regime' after armed police seized an entire archive last week detailing repression in the Soviet Union.

Figes, professor of history at Birkbeck, a London University college, condemned the raid on Memorial, a Russian human rights organisation. He said that the police had also taken material used in his latest book, The Whisperers, which details family life in Stalin's Russia.

On Thursday, armed and masked men from the investigative committee of the Russian general prosecutor's office burst into Memorial's St Petersburg office.

After a search of several hours, they confiscated its entire archive - memoirs, photographs, interviews, and other unique documents detailing the history of the gulag and the names of many of its victims.

Yesterday Figes claimed the raid 'was clearly intended to intimidate Memorial'. The confiscated archive included unique documents detailing the 'Soviet terror from 1917 to the 1960s,' he said, adding that the office was 'an important centre for historical research' and a 'voice for tens of thousands of victims of repression in Leningrad'. He said he believed the raid was 'a serious challenge to freedom of expression' in Russia: 'It is part of a campaign to rewrite Soviet history and rehabilitate the Stalinist regime.'

Memorial is Russia's oldest and best-known human rights organisation. It has pioneered research into Soviet-era repression and collaborated with Figes on his latest book, which was published last year, by interviewing dozens of elderly survivors of Stalinism and recording their personal accounts of life under tyranny.

The Whisperers: Private Lives in Stalin's Russia includes gripping testimonies of ordinary Russians who were children in the 1930s. It details their cramped living conditions, the fear of informers and the abrupt disappearance of parents, many of whom never returned from Stalin's camps.

'We were brought up to keep our mouths shut ... We went through life afraid to talk,' one woman, whose father was arrested in 1936, told the historian. 'Our mother used to say that every other person was an informer. We were afraid of our neighbours, and especially of the police. Even today, if I see a policeman I shake with fear.'

The book also recounts improbable tales of survival. In many cases, the children of arrested parents were split up and sent to different orphanages. Heroically, grandparents tried to track their grandchildren down. Some succeeded, with families eventually reunited; others did not.

Children of 'enemies of the people' also developed strategies for survival, lying about their parents' arrest and inventing fake biographies. Others wrote faithfully to their parents in camps - only to discover on their return that they had been unable to communicate with each other.

Figes estimates that between 1928, when Stalin seized control of the Communist party, and his death in 1953, the dictator repressed at least 25 million people - an eighth of the Soviet Union's population. His victims were executed, sent to gulags, deported or forced to work as slave labourers.

Last night, Figes - who is also the author of an acclaimed book on the Russian revolution, A People's Tragedy - said that police had carted off all the material used in The Whisperers. 'Luckily I have copies, and the originals of the sound recordings [interviews],' he said.

The St Petersburg centre is not connected to the human rights work of the organisation in Moscow, which has angered the Kremlin with its regular reports on abuses in Chechnya and has also recently carried out an independent investigation into the war in Georgia. None the less, the centre's work has been at odds with official attempts - which are supported by Russia's nationalists and communists - to rehabilitate Stalin. History textbooks now portray him as a great leader who industrialised the Soviet Union and saved it from the Nazis, rather than as one of the 20th century's biggest mass murderers.

Human rights groups said they were dismayed by the attack on Memorial. 'This outrageous police raid shows the poisonous climate for non-governmental organisations in Russia,' said Allison Gill, director of Human Rights Watch in Moscow, said. 'This is an overt attempt by the Russian government ... to silence critical voices.'

Staff at Memorial said it was unclear whether they would get their data back. The archive had been stored on hard drives and CDs. 'All these years of work are now in the hands of investigators and we do not know when and in what condition it will be returned,' staff member Yuri Rybakov said. He added: 'This is an act of repression against our organisation.'

A spokesman for the Russian prosecutor general's investigative unit said that the raid was part of an investigation into an article that incited racial hatred published in the Novy Peterburg newspaper in June 2007. Memorial insists that it has no connection with either the article or its author.